Ceramic particles for use in a solar power tower

ABSTRACT

Ceramic particles for use in a solar power tower and methods for making and using the ceramic particles are disclosed. The ceramic particle can include a sintered ceramic material formed from a mixture of a raw material and MnO. The sintered ceramic material can include about 0.01 wt % to about 10 wt % MnO, about 0.1 wt % to about 20 wt % Fe2O3, and about 0.01 wt % to about 10 wt % Mn2O3. The ceramic particle can have a size from about 8 mesh to about 170 mesh.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a Continuation of U.S. non-provisional patentapplication Ser. No. 16/167,317, which is a Continuation of U.S.non-provisional patent application Ser. No. 15/370,978 filed Dec. 6,2016, and which claims the benefit of U.S. patent application Ser. No.62/264,010, filed Dec. 7, 2015. The aforementioned related patentapplications are herein incorporated by reference in their entirety.

FIELD

Embodiments of the present disclosure relate generally to ceramicparticles, and more particularly to ceramic particles used in solarpower towers.

BACKGROUND

Solar power towers are towers used to receive focused sunlight reflectedby a plurality of movable mirrors, or heliostats, such as those locatedin a heliostat solar power plant. These towers oftentimes contain solarabsorption media such as falling liquids, ceramic particles, or sandthat can capture the reflected sunlight in the form of thermal energy.The thermal energy contained in the solar absorption media is then usedto generate electricity. In many cases, the thermal energy contained inthe solar absorption media is transferred to water to generate steamused to drive turbines which produce electricity resulting in cooledsolar absorption media. The cooled solar absorption media is typicallyrecycled for repeated exposure to the reflected sunlight in the solarpower tower.

It has been found that dark ceramic particles tend to enhance solarenergy absorption properties. However, it has also been found that asthese dark particles are repeatedly exposed to elevated solartemperatures, which can exceed 500° C., they become lighter in color andas a result lose some of their solar absorption capacity. There is aneed, therefore, for ceramic particles that can maintain their solarabsorption capacity after repeated exposure to solar energy.

BRIEF DESCRIPTION OF THE DRAWINGS

The present disclosure may best be understood by referring to thefollowing description and accompanying drawings that are used toillustrate embodiments of the present disclosure. In the drawings:

FIG. 1 is a schematic illustration of a system for preparing ceramicparticles from a slurry as described herein.

FIG. 2 is a schematic illustration of a drip cast system for preparingceramic particles from a slurry as described herein.

FIG. 3 shows a schematic illustration of an elevation view of a solarpower tower in a field of heliostats.

FIG. 4 shows a schematic illustration of a solid particle process flowloop for a solar power tower.

DETAILED DESCRIPTION

In the following description, numerous specific details are set forth.However, it is understood that embodiments of the present disclosure maybe practiced without these specific details. In other instances,well-known structures and techniques have not been shown in detail inorder not to obscure the understanding of this description.

Described herein are ceramic particles capable of absorbing solarradiation. In particular, ceramic particles for use in solar powertowers are described herein. Also described herein are methods formaking ceramic particles having an increased solar absorption capacity.In particular, methods of incorporating manganese oxide into ceramicparticles are described herein. Also described herein are methods ofusing ceramic particles in a solar power tower.

The ceramic particles disclosed herein can be or include darkenedceramic particles. The darkened ceramic particles can have a darkeningcomponent. The darkening component can be or include any colorantmaterial suitable for darkening the ceramic particles. The darkeningcomponent can be or include any one or more metal oxides suitable fordarkening the ceramic particles. In one or more exemplary embodiments,the darkening component can be or include one or more transition metaloxides. For example, the darkening component can be or include one ormore oxides of iron, cobalt, manganese, magnesium, nickel, molybdenum,or tungsten. In one or more exemplary embodiments, the darkeningcomponent can be or include iron oxide and/or manganese oxide. In one ormore exemplary embodiments, manganese oxide can be selected from one ormore of MnO, Mn₂O₃, and MnO₂ and any mixture thereof. For example, thedarkening component can be manganese oxide in the Mn²⁺ state (MnO)and/or manganese oxide in the Mn²⁺ state (Mn₂O₃).

In has also been found that iron oxide in the Fe²⁺ state (FeO), which isa dark colored compound, can lighten when exposed to elevatedtemperatures, such as those present in the solar power tower. In hasalso been found that MnO, which is a light colored compound, can darkenwhen exposed to the elevated temperatures present in the solar powertower. For example, upon thermal exposure in air, iron and manganese canoxidize from 2⁺ to 3⁺. The shift from Fe²⁺ to Fe³⁺ (FeO to Fe₂O₃)lightens the Fe component and thus can lighten the coloration of theceramic particles containing the Fe component. In contrast, the shiftfrom Mn²⁺ to Mn²⁺ (MnO to Mn₂O₃) darkens the Mn component and can darkenthe coloration of the ceramic particles containing the Mn component.

The darkening component can be added to or incorporated into the ceramicparticle in any suitable manner. In one or more exemplary embodiments,the darkening component can be added to a sintered ceramic particle,during any stage in a manufacturing process used to produce the ceramicparticle, or to raw materials used to produce the ceramic particle, orany combination thereof. In one or more exemplary embodiments, manganeseoxide can be combined with any suitable raw ceramic feedstock prior tobeing introduced to a manufacturing process suitable to produce theceramic particles. Suitable raw ceramic feedstocks can include, but arenot limited to, alumina, silica, zirconia, zinc oxide, silicon nitride,silicon carbide, fly ash, and naturally occurring clays, such as kaolinand/or bauxite, and the like and any combinations thereof. Suitablemanufacturing process include, but are not limited to, continuous sprayatomization, spray fluidization, drip casting, spray drying, orcompression. Suitable ceramic particles and methods for manufacture aredisclosed in U.S. Pat. Nos. 4,068,718, 4,427,068, 5,188,175, 7,036,591,8,865,631, 8,883,693, and 9,175,210, and U.S. patent application Ser.Nos. 14/502,483 and 14/802,761, the entire disclosures of which areincorporated herein by reference, the entire disclosures of which areincorporated herein by reference.

In one or more exemplary embodiments, the darkening component can beadded to a ceramic particle in its method of manufacture. The ceramicparticles can be made according to a method as described in U.S. Pat.No. 4,879,181, the entire disclosure of which is incorporated herein byreference. The ceramic raw material can be initially calcined in acalciner by any suitable calcining method at temperatures and timessufficiently high to remove organic material and to substantially removewater of hydration. The calcined ceramic raw material can be added in apredetermined ratio to a high intensity mixer. In one or more exemplaryembodiments, at least about 40% of the ceramic raw material on a dryweight basis is clay. The calcined ceramic raw material can have anaverage particle size of less than about 15 microns, less than about 10microns, less than about 5 microns, or between about 3 microns and 0.5microns.

The calcined ceramic raw material can be stirred to form a dryhomogeneous particulate mixture having an average particle size of lessthan about 15 microns. A suitable stirring or mixing device is thatobtainable from Eirich Machines, Inc., known as the Eirich Mixer. Amixer of this type can be provided with a horizontal or inclinedcircular table, which can be made to rotate at a speed of from about 10to about 60 revolutions per minute (rpm), and can be provided with arotatable impacting impeller, which can be made to rotate at a tip speedof from about 5 to about 50 meters per second. The direction of rotationof the table can be opposite that of the impeller, causing materialadded to the mixer to flow over itself in countercurrent manner. Thecentral axis of the impacting impeller can be located within the mixerat a position off center from the central axis of the rotatable table.The table can be in a horizontal or inclined position, wherein theincline, if any, can be between 0 and 35 degrees from the horizontal.

While the mixture is being stirred, a suitable amount of water can beadded to cause formation of composite, spherical pellets from theceramic powder mixture. The total quantity of water sufficient to causeessentially spherical pellets to form can be from about 17 to about 20wt % of the calcined ceramic raw material. The total mixing time can befrom about 2 to about 6 minutes.

After the clay mixture is added to the mixer, the table can be rotatedat from about 10 to about 60 rpm or from about 20 to about 40 rpm, andthe impacting impeller can be rotated to obtain a tip speed of fromabout 25 to about 50 or from about 25 to about 35, meters per second,and sufficient water can be added to cause essentially spherical pelletsof the desired size to form. If desired, the impeller can be initiallyrotated at from about 5 to about 20 meters per second during addition ofone-half of the sufficient water and subsequently rotated at the highertip speed of 25 to about 50 meters per second during the addition of thebalance of the water. The rate of water addition is not critical. Theintense mixing action can quickly disperse the water throughout theparticles.

The resulting pellets can be dried at a temperature of between about100° C. (212° F.) and about 300° C. (572° F.) until less than 3 percentor less than 1 percent moisture remains in the pellets. For example, thedrying temperature can be between about 175° C. (347° F.) and 275° C.(527° F.), and the drying time can be between about 30 and about 60minutes.

The dried pellets can then be furnaced at a sintering temperature for aperiod sufficient to enable recovery of the ceramic particles. Thespecific time and temperature to be employed can be dependent on thestarting ingredients and can be determined empirically according to theresults of physical testing of ceramic particles after furnacing.Furnacing can be carried out to sinter the composite pellets; generally,temperatures of between about 1,250° C. and about 1,550° C. for about 4to about 20 minutes or from about 1,400° C. to about 1,515° C. for about4 to about 8 minutes.

The darkening component can be added at any suitable stage in the methoddescribed above. In one or more exemplary embodiments, the darkeningcomponent can be introduced at any location prior to, on, or after, thecalciner and/or the Eirich Mixer to provide the ceramic particle.

Referring now to FIG. 1 , an exemplary system for implementing acontinuous process for preparing sintered, substantially round andspherical particles from a slurry is illustrated. The exemplary systemillustrated in FIG. 1 is similar in configuration and operation to thatdescribed in U.S. Pat. No. 4,440,866, the entire disclosure of which isincorporated herein by reference.

In the system illustrated in FIG. 1 , a ceramic raw material is passedthrough a shredder 105 which slices and breaks apart the raw materialinto small chunks. The ceramic raw material can have any suitablealumina content. For example, the ceramic raw material can have analumina content of about 10 wt %, about 20 wt %, about 30 wt %, or about40 wt % to about 50 wt %, about 60 wt %, about 70 wt %, about 80 wt %,about 90 wt %, or about 95 wt % or more. In some embodiments, when theraw material as mined, or as received, (referred to herein as“untreated” raw material) is of such consistency that it can beprocessed as described herein without shredding, the shredder may bebypassed. Raw material fed through a shredder such as is illustrated inFIG. 1 , is referred to as “treated” raw material.

In certain embodiments, the shredder breaks apart and slices the rawmaterial so as to yield pieces having a diameter of less than about fiveinches, although pieces having smaller and larger diameters can befurther processed into a slurry as described herein. Shredders andnumerous other devices for slicing, chopping or comminuting the rawmaterial, as well as commercial sources for same, such as the GleasonFoundry Company, are well-known to those of ordinary skill in the art.

The treated or untreated raw material and water are fed to a blunger110, which has a rotating blade that imparts a shear force to andfurther reduces the particle size of the raw material to form a slurry.In a continuous process, the raw material and water are continuously fedto the blunger. Blungers and similar devices for making slurries of suchmaterials, as well as commercial sources for same are well-known tothose of ordinary skill in the art.

In certain embodiments, the darkening component is added to the rawmaterial and water in the blunger 110 to result in a darkening componentconcentration of about 1 ppm, about 10 ppm, about 50 ppm, about 0.01%,about 0.05%, about 0.1%, about 0.5%, or about 1% to about 2%, about 3%,about 5%, about 7.5%, about 10%, about 15%, or about 20% or more byweight of the solids content in the slurry or just prior to theformation of pellets as described below.

A sufficient amount of water is added to the blunger 110 to result in aslurry having a solids content in the range of from about 40% to about60% by weight. In certain embodiments, a sufficient amount of water isadded to the slurry such that the solids content of the slurry is fromabout 45% to about 55% by weight. In still other embodiments, asufficient amount of water is added to the slurry such that the solidscontent of the slurry is about 50% by weight. The water added to theblunger 110 can be fresh water or deionized water. In a continuousprocess for preparing the slurry, the solids content of the slurry isperiodically analyzed and the amount of water fed to the slurry adjustedto maintain the desired solids content. Methods for analyzing the solidscontent of a slurry and adjusting a feed of water are well-known andunderstood by those of ordinary skill in the art.

In certain embodiments, a dispersant is added to the slurry in theblunger 110 to adjust the viscosity of the slurry to a target range asdiscussed further below. In other embodiments, the viscosity of theslurry in the blunger 110 is adjusted to the target range by theaddition of a dispersant and a pH-adjusting reagent.

A dispersant may be added to the slurry prior to the addition of thedarkening material or other additives. In certain embodiments, thecomposition includes a dispersant in an amount of from about 0.15% toabout 0.30% by weight based on the dry weight of the raw material.

Exemplary materials suitable for use as a dispersant in the compositionsand methods described herein include but are not limited to sodiumpolyacrylate, ammonium polyacrylate, ammonium polymethacrylate, tetrasodium pyrophosphate, tetra potassium pyrophosphate, polyphosphate,ammonium polyphosphate, ammonium citrate, ferric ammonium citrate, andpolyelectrolytes such as a composition of ammonium polymethacrylate andwater commercially available from a variety of sources, such as, KemiraChemicals under the trade name C-211, Phoenix Chemicals, Bulk ChemicalSystems under the trade name BCS 4020 and R.T. Vanderbilt Company, Inc.under the trade name DARVAN C. Generally, the dispersant can be anymaterial that will adjust the viscosity of the slurry to a targetviscosity such that the slurry can be subsequently processed through oneor more pressure nozzles of a fluidizer. In certain embodiments, thetarget viscosity is less than 150 centipoises (cps) (as determined on aBrookfield Viscometer with a #61 spindle). In other embodiments, thetarget viscosity is less than 100 cps.

According to embodiments in which a pH-adjusting reagent is used, asufficient amount of a pH-adjusting reagent is added to the slurry toadjust the pH of the slurry to a range of from about 8 to about 11. Incertain embodiments, a sufficient amount of the pH-adjusting reagent isadded to the slurry to adjust the pH to about 9, about 9.5, about 10 orabout 10.5. The pH of the slurry can be periodically analyzed by a pHmeter, and the amount of pH-adjusting reagent fed to the slurry adjustedto maintain a desired pH. Methods for analyzing the pH of a slurry andadjusting the feed of the pH-adjusting reagent are within the ability ofthose of ordinary skill in the art. Exemplary materials suitable for useas a pH-adjusting reagent in the compositions and methods describedherein include but are not limited to ammonia and sodium carbonate.

Generally, the target viscosity of the compositions is a viscosity thatcan be processed through a given type and size of pressure nozzle in afluidizer, without becoming clogged. Generally, the lower the viscosityof the slurry, the more easily it can be processed through a givenfluidizer. However, the addition of too much dispersant can cause theviscosity of the slurry to increase to a point that it cannot besatisfactorily processed through a given fluidizer. One of ordinaryskill in the art can determine the target viscosity for given fluidizertypes through routine experimentation.

The blunger 110 can mix the raw material, darkening component, water,dispersant and pH-adjusting reagent until a slurry is formed. The lengthof time required to form a slurry is dependent on factors such as thesize of the blunger, the speed at which the blunger is operating, andthe amount of material in the blunger.

From the blunger 110, the slurry is fed to a tank 115, where the slurryis continuously stirred, and a binder is added in an amount of fromabout 0.2% to about 5.0% by weight, based on the total dry weight of theraw material and the darkening component. In certain embodiments, thebinder is added in an amount of from about 0.2% to about 3.0% by weightbased on the total dry weight of the raw material and the darkeningcomponent. Suitable binders include but are not limited to polyvinylacetate, polyvinyl alcohol (PVA), methylcellulose, dextrin,pregelatanized corn starch, pregelatanized potato starch, and molasses.In certain embodiments, the binder is PVA having a molecular weight offrom about 20,000 to 100,000 M_(n). “M_(n)” represents the numberaverage molecular weight which is the total weight of the polymericmolecules in a sample, divided by the total number of polymericmolecules in that sample.

The tank 115 maintains the slurry created by the blunger 110. However,the tank 115 stirs the slurry with less agitation than the blunger, soas to mix the binder with the slurry without causing excessive foamingof the slurry or increasing the viscosity of the slurry to an extentthat would prevent the slurry from being fed through the pressurizednozzles of a fluidizer.

In another embodiment, the binder can be added to the slurry while inthe blunger. In this embodiment, the blunger optionally has variablespeeds, including a high speed to achieve the high intensity mixing forbreaking down the raw material into a slurry form, and a low speed tomix the binder with the slurry without causing the above-mentionedexcessive foaming or increase in viscosity.

Referring again to the tank 115 illustrated in FIG. 1 , the slurry isstirred in the tank, after addition of the binder, for a time sufficientto thoroughly mix the binder with the slurry. In certain embodiments,the slurry is stirred in the tank for up to about 30 minutes followingthe addition of binder. In other embodiments, the slurry is stirred inthe tank 115 for at least about 30 minutes. In still other embodiments,the slurry is stirred in the tank for more than about 30 minutes afteraddition of the binder.

Tank 115 can also be a tank system comprised of one, two, three or moretanks. Any configuration or number of tanks that enables the thoroughmixing of the binder with the slurry is sufficient. In a continuousprocess, water, and one or more of dust, oversized particles, orundersized particles from a subsequent fluidizer or other apparatus canbe added to the slurry in the tank 115. From the tank 115, the slurry isfed to a heat exchanger 120, which heats the slurry to a temperature offrom about 25° C. to about 90° C. From the heat exchanger 120, theslurry is fed to a pump system 125, which feeds the slurry, underpressure, to a fluidizer 130.

A grinding mill(s) and/or a screening system(s) (not illustrated) can beinserted at one or more places in the system illustrated in FIG. 1 priorto feeding the slurry to the fluidizer to assist in breaking anylarger-sized raw material down to a target size suitable for feeding tothe fluidizer. In certain embodiments, the target size is less than 230mesh. In other embodiments, the target size is less than 325 mesh, lessthan 270 mesh, less than 200 mesh or less than 170 mesh. The target sizeis influenced by the ability of the type and/or size of the pressurenozzle in the subsequent fluidizer to atomize the slurry withoutbecoming clogged.

If a grinding system is employed, it is charged with a grinding mediasuitable to assist in breaking the raw material down to a target sizesuitable for subsequent feeding through one or more pressure nozzles ofa fluidizer. If a screening system is employed, the screening system isdesigned to remove particles larger than the target size from theslurry. For example, the screening system can include one or morescreens, which are selected and positioned so as to screen the slurry toparticles that are smaller than the target size.

Referring again to FIG. 1 , fluidizer 130 is of conventional design,such as described in, for example, U.S. Pat. No. 3,533,829 and U.K. Pat.No. 1,401,303. Fluidizer 130 includes at least one atomizing nozzle 132(three atomizing nozzles 132 being shown in FIG. 1 ), which is apressure nozzle of conventional design. In other embodiments, one ormore two-fluid nozzles are suitable. The design of such nozzles is wellknown, for example from K. Masters: “Spray Drying Handbook”, John Wileyand Sons, New York (1979).

Fluidizer 130 further includes a particle bed 134, which is supported bya plate 136, such as a perforated, straight or directional plate. Hotair flows through the plate 136. The particle bed 134 comprises seedsfrom which green pellets of a target size can be grown. The term “greenpellets” and related forms, as used herein, refers to substantiallyround and spherical particles which have been formed from the slurry butare not sintered. When a perforated or straight plate is used, the seedsalso serve to obtain plug flow in the fluidizer. Plug flow is a termknown to those of ordinary skill in the art, and can generally bedescribed as a flow pattern where very little back mixing occurs. Theseed particles are smaller than the target size for green pellets madeaccording to the present methods. In certain embodiments, the seedcomprises from about 5% to about 20% of the total volume of a greenpellet formed therefrom. Slurry is sprayed, under pressure, through theatomizing nozzles 132, and the slurry spray coats the seeds to formgreen pellets that are substantially round and spherical.

External seeds can be placed on the perforated plate 136 beforeatomization of the slurry by the fluidizer begins. If external seeds areused, the seeds can be prepared in a slurry process similar to thatillustrated in FIG. 1 , where the seeds are simply taken from thefluidizer at a target seed size. External seeds can also be prepared ina high intensity mixing process such as that described in U.S. Pat. No.4,879,181, the entire disclosure of which is hereby incorporated byreference.

According to certain embodiments, external seeds are made from either araw material having at least the same alumina content as the rawmaterial used to make the slurry, or from a raw material having more orless alumina than the raw material used to make the slurry. In certainembodiments, the slurry has an alumina content that is at least 10%, atleast 20%, or at least 30% less than that of the seeds. In otherembodiments, the external seeds have an alumina content less than thatof the slurry, such as at least 10%, at least 20%, or at least 30% lessthan that of the slurry.

Alternatively, seeds for the particle bed are formed by the atomizationof the slurry, thereby providing a method by which the slurry“self-germinates” with its own seed. According to one such embodiment,the slurry is fed through the fluidizer 130 in the absence of a seededparticle bed 134. The slurry droplets exiting the nozzles 132 solidify,but are small enough initially that they get carried out of thefluidizer 130 by air flow and caught as “dust” (fine particles) by adust collector 145, which may, for instance, be an electrostaticprecipitator, a cyclone, a bag filter, a wet scrubber or a combinationthereof. The dust from the dust collector is then fed to the particlebed 134 through dust inlet 162, where it is sprayed with slurry exitingthe nozzles 132. The dust may be recycled a sufficient number of times,until it has grown to a point where it is too large to be carried out bythe air flow and can serve as seed. The dust can also be recycled toanother operation in the process, for example, the tank 115.

Referring again to FIG. 1 , hot air is introduced to the fluidizer 130by means of a fan and an air heater, which are schematically representedat 138. The velocity of the hot air passing through the particle bed 134is from about 0.9 meters/second to about 1.5 meters/second, and thedepth of the particle bed 134 is from about 2 centimeters to about 60centimeters. The temperature of the hot air when introduced to thefluidizer 130 is from about 250° C. to about 650° C. The temperature ofthe hot air as it exits from the fluidizer 130 is less than about 250°C., and in some embodiments is less than about 100° C.

The distance between the atomizing nozzles 132 and the plate 136 isconfigured to avoid the formation of dust which occurs when the nozzles132 are too far away from the plate 126 and the formation of irregular,coarse particles which occurs when the nozzles 132 are too close to theplate 136. The position of the nozzles 132 with respect to the plate 136is adjusted on the basis of an analysis of powder sampled from thefluidizer 130.

The green pellets formed by the fluidizer accumulate in the particle bed134. In a continuous process, the green pellets formed by the fluidizer130 are withdrawn through an outlet 140 in response to the level ofproduct in the particle bed 134 in the fluidizer 130, so as to maintaina given depth in the particle bed. A rotary valve 150 conducts greenpellets withdrawn from the fluidizer 130 to an elevator 155, which feedsthe green pellets to a screening system 160, where the green pellets areseparated into one or more fractions, for example, an oversizedfraction, a product fraction, and an undersized fraction.

The oversized fraction exiting the screening unit 160 includes thosegreen pellets that are larger than the desired product size. In acontinuous process, the oversized green pellets may be recycled to tank115, where at least some of the oversized green pellets can be brokendown and blended with slurry in the tank. Alternatively, oversized greenpellets can be broken down and recycled to the particle bed 134 in thefluidizer 130. The undersized fraction exiting the screening system 160includes those green pellets that are smaller than the desired productsize. In a continuous process, these green pellets may be recycled tothe fluidizer 130, where they can be fed through an inlet 162 as seedsor as a secondary feed to the fluidizer 130.

The product fraction exiting the screening system 160 includes thosegreen pellets having the desired product size. These green pellets aresent to a pre-sintering device 165, for example, a calciner, where thegreen pellets are dried or calcined prior to sintering. In certainembodiments, the green pellets are dried to a moisture content of lessthan about 18% by weight, or less than about 15% by weight, about 12% byweight, about 10% by weight, about 5% by weight, or about 1% by weight.

After drying and/or calcining, the green pellets are fed to a sinteringdevice 170, in which the green pellets are sintered for a period of timesufficient to enable recovery of sintered, substantially round andspherical particles having one or more of a desired density, bulkdensity, and crush strength. Alternatively, the pre-sintering device 165can be eliminated if the sintering device 170 can provide sufficientcalcining and/or drying conditions (i.e., drying times and temperaturesthat dry the green pellets to a target moisture content prior tosintering), followed by sufficient sintering conditions.

The specific time and temperature to be employed for sintering isdependent on the starting ingredients and the desired density for thesintered particles. In some embodiments, sintering device 170 is arotary kiln, operating at a temperature of from about 1000° C. to about1600° C., for a period of time from about 5 to about 90 minutes. Incertain embodiments, a rotary kiln is operated at a temperature of about1000° C., about 1200° C., about 1300° C., about 1400° C. or about 1500°C. In certain embodiments, the green pellets have a residence time inthe sintering device of from about 50 minutes to about 70 minutes, orfrom about 30 minutes to about 45 minutes. After the particles exit thesintering device 170, they can be further screened for size, and testedfor quality control purposes.

The darkening component can be added at any suitable stage in the systemillustrated in FIG. 1 . In one or more exemplary embodiments, thedarkening component can be introduced to the system illustrated in FIG.1 at any location prior to, on, or after, the shredder 105, the blunger110, the tank 115, the heat exchanger 120, the pump system 125, and/orbefore the fluidizer 130 to provide the ceramic particle.

FIG. 2 is a schematic illustration of a drip cast system for preparingceramic particles from a slurry as described herein. As shown in FIG. 2, a ceramic raw material is passed through the shredder 105 and fed tothe blunger 110 where the ceramic raw material, darkening component,water, dispersant, and/or pH-adjusting reagent can be mixed until aslurry is formed. From the blunger 110, the slurry is fed to the tank115, where the slurry is continuously stirred and the binder is added.From the tank 115, the slurry is fed to the heat exchanger 120, whichheats the slurry to a desired temperature. From the heat exchanger 120,the slurry is fed to the pump system 125, which feeds the slurry to afeed tank 702. A nozzle 704 receives a slurry from the feed tank 704,which contains the ceramic raw materials suspended in water or any othersuitable aqueous solution. Pressure applied to feed tank 702 by apressure supply system (not shown) causes the slurry to flow throughnozzle 704 at a selected rate to form droplets. Below nozzle 704 is acoagulation vessel 706, which receives the droplets. A vibrator unit(not shown) is connected to the nozzle 704 and is used to supplypressure pulses to the nozzle or directly in the slurry flowing to thenozzle 704. The resulting vibration of the slurry flow through thenozzle 704 causes the stream exiting the nozzle 704 to break intodroplets of uniform size as the droplets fall from the nozzle 704 andinto an atmosphere surrounding the nozzle 704. The surroundingatmosphere can include any suitable gaseous medium, such as air ornitrogen. As droplets fall toward coagulation vessel 706, surfacetension effects tend to form the droplets into spheres. These fallingdroplets, or spheres, then contact an upper liquid surface of acoagulation liquid contained in the coagulation vessel 706. The dropletssolidify and form into green pellets in the coagulation liquid. Thegreen pellets formed in the coagulation vessel are thus formed withoutthe necessity of a sol-gel reaction, reaction gas free fall zone, foamedlayer of reaction liquid or reaction liquid directed onto the dropletsprior to entering the reaction liquid bath.

The slurry in the feed tank 702 can have any suitable solids content.The solids content of the slurry can range from about 15%, about 20%,about 25%, or about 35% to about 55%, about 65%, about 75%, or about85%. In one or more exemplary embodiments, the solids content can befrom about 25% to about 75%. The viscosity of the slurry can be fromabout 1, about 10, about 25, about 50, about 100, or about 250 to about500, about 750, about 1,000, about 2,500 centipoise (cP) or more.Adjusting the viscosity of the slurry can aid in improving dropletformation and formation of spherical particles. The viscosity of theslurry can be optimized or adjusted via selection of reactant typeand/or reactant concentration. Optimization of the dispersant type andconcentration can also reduce the viscosity of the slurry. Dispersantscan be selected based on cost, availability and effectiveness inreducing the viscosity of a selected slurry. Dispersants that can beused to reduce the viscosity of slurry include sodium silicate, ammoniumpolyacrylate, sodium polymethacrylate, sodium citrate, sodiumpolysulfonate and hexametaphosphate.

The slurry in the feed tank 702 can also contain any suitable reactantthat will react with a component in coagulation liquid in coagulationvessel 706 to form a semi-solid or insoluble compound. This reactant canbe or include a monosaccharide, a disaccharide, a polysaccharide, citricacid, methylcellulose, polyvinyl alcohol, polyvinyl acetate, or boratefluids or any combination or mixture thereof. In one or more exemplaryembodiments, the reactant is a polysaccharide, such as sodium alginate.Sodium alginate is a naturally occurring polysaccharide that is solublein water as the sodium salt but is cross-linked to form a gel as thecalcium salt.

In one or more exemplary embodiments, the reactant can be or include anysuitable polymer or co-polymer with a divalent exchange mechanism. Thereactant can be or include poly(ethylene oxide), ethylene-vinyl acetatecopolymers, carboxylic acid polymers and copolymers, acrylate polymersand copolymers, and methacrylate polymers and copolymers. In one or moreexemplary embodiments, the reactant can be or include any suitabledivalent polymer or co-polymer. In one or more exemplary embodiments,the reactant can be or include poly(maleic acid) (PMA), poly(acrylicacid) (PAA), or any combination thereof. For example, the reactant canbe or include a PMA:PAA copolymer.

The slurry can include the reactant in any suitable amounts. The slurrycan have a reactant concentration of about 0.01 wt %, about 0.05 wt %,about 0.1 wt %, about 0.25 wt %, about 0.5 wt %, about 0.8 wt %, about1.2 wt %, or about 1.5 wt % to about 1.8 wt %, about 2 wt %, about 2.5wt %, about 3 wt %, about 4 wt %, about 5 wt %, about 6 wt %, or about 8wt %. In one or more exemplary embodiments, the slurry reactantconcentration can be from about 0.2 wt % to about 4 wt %, about 0.4 wt %to about 2.8 wt %, about 0.6 wt % to about 2.4 wt %, about 0.8 wt % toabout 1.8 wt %, or about 1.2 wt % to about 1.6 wt %.

In certain embodiments, the darkening component is added to the rawmaterial and water in the slurry to result in a darkening componentconcentration of about 1 ppm, about 10 ppm, about 50 ppm, about 0.01%,about 0.05%, about 0.1%, about 0.5%, or about 1% to about 2%, about 3%,about 5%, about 7.5%, about 10%, about 15%, or about 20% or more byweight of the solids content in the slurry or just prior to theformation of pellets as described below.

Coagulation tank 706 can contain a coagulation liquid which gels thereactant chemical in the slurry. In other words, the coagulation liquidcan include any suitable coagulation agent which gels the reactant. Thecoagulation agent can also be or include any cationic material suitablefor ion exchange with the reactant. The coagulation agent can be orinclude a divalent, trivalent or higher cationic material. In one ormore exemplary embodiments, the coagulation agent can be or include oneor more salts of calcium, magnesium, strontium, aluminum, and/or iron.For example, the coagulation agent can be or include one or more ofcalcium chloride, magnesium chloride, or the like. The coagulationliquid can be or include an aqueous solution containing the coagulationagent. The coagulation liquid can have a coagulation agent concentrationof about 0.01 wt %, about 0.05 wt %, about 0.1 wt %, about 0.5 wt %,about 1 wt %, about 2 wt %, or about 4 wt % to about 6 wt %, about 8 wt%, about 10 wt %, about 15 wt %, or about 20 wt % or more. In one ormore exemplary embodiments, a coagulation liquid for sodium alginate isa calcium chloride solution at concentration levels of 0.5% to 10% byweight.

In one or more exemplary embodiments, the coagulation liquid in thecoagulation tank 706 can contain a coagulation agent, a reducing agentand/or the darkening component. In one or more exemplary embodiments,the slurry disclosed herein can also contain a reducing agent and/or thedarkening component.

The diameter of nozzle 704, the viscosity of slurry, the ceramicparticle content of slurry, pressure to feed the slurry to the nozzle,along with the frequency and amplitude of vibration applied by vibratorsource are adjusted to produce droplets having a desired size. Thesevariables are preferably set at a constant value as spheres are producedto be formed into a batch of pellets of propping material. Differentbatches may be produced having different size pellets. Preferably, eachbatch will be monosized (i.e., contained on a single sieve such aspassing through a 20 mesh sieve but staying on a 25 mesh sieve). Thepressure used to feed slurry to the nozzle is adjusted to create laminarflow through the nozzle. The feed pressure can range from 1 to 50 psi.The frequency is adjusted for each set of slurry conditions such that aresonance is established in the slurry stream exiting the nozzle thatthen produces spherical droplets. The frequency can range from 10 to20,000 Hz. The pressure and frequency are optimized iteratively tocreate uniform spherical shapes. The amplitude is adjusted to improvethe uniform shape of the spherical droplets formed. The flow rate of theslurry through a nozzle is a function of the nozzle diameter, slurryfeed pressure, and the slurry properties such as viscosity and density.For example, for kaolin and alumina slurries through nozzles up to 500microns in diameter the flow rate per nozzle can range from 0.2 to 3kg/hr, which equates to a mass flux of about 1 to about 15 kg/(mm²×hr).

The distance between nozzle 704 and the top of the liquid in coagulationvessel 706 is selected to allow droplets to become spherical beforereaching the top of the liquid. The distance can be from 1 to 20 cm, butis more typically in the range of 1 to 5 cm so as to reduce distortionof the droplet shape upon impact with the liquid surface, therebyeliminating the need for a reaction gas, foam layer, or tangentiallydirected reaction liquid prior to the droplets entering the coagulationvessel 706. The reactant chemical in the droplets of slurry reacts withthe coagulation liquid in the coagulation vessel 706 and a semi-solidsurface is formed on the droplets, which helps retain the sphericalshape and prevents agglomeration of the pellets. Preferably, theresidence time of pellets in coagulation vessel 706 is sufficient toallow pellets to become rigid enough to prevent deformation of thespherical shape when they are removed and dried, i.e., semi-rigid. Insome embodiments, pellets may fall into a coagulation liquid solutionflowing vertically upward so that settling of the particle through theliquid will be retarded to produce a longer residence time in thecoagulation vessel 706.

Green pellets formed using the drip cast system of FIG. 2 can be washedto remove excess coagulation agent and conveyed to other devices such asthe pre-sintering device 165 and/or the sintering device 170. Thedarkening component can be added at any suitable stage in the systemillustrated in FIG. 2 . In one or more exemplary embodiments, thedarkening component can be introduced to the system illustrated in FIG.2 , at any location prior to, on, or after, the shredder 105, theblunger 110, the tank 115, the heat exchanger 120, the pump system 125,the feed tank 702, the coagulation vessel 706, the pre-sintering device165, and/or the sintering device 170 to provide the ceramic particle.

The ceramic particle can have any suitable composition. In one or moreexemplary embodiments, the ceramic particle can be or include silicaand/or alumina in any suitable amounts. According to several exemplaryembodiments, the ceramic particle includes less than 80 wt %, less than60 wt %, less than 40 wt %, less than 30 wt %, less than 20 wt %, lessthan 10 wt %, or less than 5 wt % silica based on the total weight ofthe ceramic particle. According to several exemplary embodiments, theceramic particle includes from about 0.1 wt % to about 70 wt % silica,from about 1 wt % to about 60 wt % silica, from about 2.5 wt % to about50 wt % silica, from about 5 wt % to about 40 wt % silica, or from about10 wt % to about 30 wt % silica. According to several exemplaryembodiments, the ceramic particle includes at least about 30 wt %, atleast about 50 wt %, at least about 60 wt %, at least about 70 wt %, atleast about 80 wt %, at least about 90 wt %, or at least about 95 wt %alumina based on the total weight of the ceramic particle. According toseveral exemplary embodiments, the ceramic particle includes from about30 wt % to about 99 wt % alumina, from about 40 wt % to about 95 wt %alumina, from about 50 wt % to about 90 wt % alumina, from about 60 wt %to about 95 wt % alumina, or from about 70 wt % to about 90 wt %alumina.

The ceramic particle can have any suitable darkening component content.In one or more exemplary embodiments, the ceramic particle has adarkening component concentration of about 1 ppmw, about 10 ppmw, about50 ppmw, about 0.01 wt %, about 0.05 wt %, about 0.1 wt %, about 0.5 wt%, or about 1 wt % to about 2 wt %, about 3 wt %, about 5 wt %, about7.5 wt %, about 10 wt %, about 15 wt %, or about 20 wt % or more basedon the total weight of the ceramic particle. In one or more exemplaryembodiments, the ceramic particle has an iron oxide concentration ofabout 2 ppmw, about 20 ppmw, about 150 ppmw, about 0.05 wt %, about 0.1wt %, about 0.5 wt %, about 1 wt %, or about 2 wt % to about 4 wt %,about 6 wt %, about 8 wt %, about 10 wt %, about 15 wt %, about 20 wt %,or about 30 wt % or more based on the total weight of the ceramicparticle. In one or more exemplary embodiments, the ceramic particle hasa MnO concentration of about 1 ppmw, about 10 ppmw, about 50 ppmw, about0.01 wt %, about 0.05 wt %, about 0.1 wt %, about 0.5 wt %, or about 1wt % to about 2 wt %, about 3 wt %, about 5 wt %, about 7.5 wt %, about10 wt %, about 15 wt %, or about 20 wt % or more based on the totalweight of the ceramic particle. In one or more exemplary embodiments,the ceramic particle has a Mn₂O₃ concentration of about 1 ppmw, about 10ppmw, about 50 ppmw, about 0.01 wt %, about 0.05 wt %, about 0.1 wt %,about 0.5 wt %, or about 1 wt % to about 2 wt %, about 3 wt %, about 5wt %, about 7.5 wt %, about 10 wt %, about 15 wt %, or about 20 wt % ormore based on the total weight of the ceramic particle.

According to several exemplary embodiments, the ceramic compositionsdisclosed herein include ceramic particles that are substantially roundand spherical having a size in a range between about 6 and 270 U.S.Mesh. For example, the size of the ceramic particle can be expressed asa grain fineness number (GFN) in a range of from about 15 to about 300,or from about 30 to about 110, or from about 40 to about 70. Accordingto such examples, a sample of ceramic particles can be screened in alaboratory for separation by size, for example, intermediate sizesbetween 20, 30, 40, 50, 70, 100, 140, 200, and 270 U.S. mesh sizes todetermine GFN. The correlation between sieve size and GFN can bedetermined according to Procedure 106-87-S of the American FoundrySociety Mold and Core Test Handbook, which is known to those of ordinaryskill in the art.

The ceramic particles can have any suitable size. For example, theceramic particle can have a mesh size of at least about 6 mesh, at leastabout 10 mesh, at least about 16 mesh, at least about 20 mesh, at leastabout 25 mesh, at least about 30 mesh, at least about 35 mesh, or atleast about 40 mesh. According to several exemplary embodiments, theceramic particle has a mesh size from about 6 mesh, about 10 mesh, about16 mesh, or about 20 mesh to about 25 mesh, about 30 mesh, about 35mesh, about 40 mesh, about 45 mesh, about 50 mesh, about 70 mesh, about100 mesh, about 140 mesh, about 170 mesh, or about 200 mesh. Accordingto several exemplary embodiments, the ceramic particle has a mesh sizefrom about 4 mesh to about 120 mesh, from about 8 mesh to about 170mesh, from about 10 mesh to about 60 mesh, from about 16 mesh to about20 mesh, from about 20 mesh to about 40 mesh, or from about 25 mesh toabout 35 mesh.

The ceramic particles disclosed herein can have any suitable shape. Theceramic particles can be substantially round, cylindrical, square,rectangular, elliptical, oval, egg-shaped, or pill-shaped. In one ormore exemplary embodiments, the ceramic particles are substantiallyround and spherical. The ceramic particles can have an averagesphericity value of about 0.5 or greater, about 0.7 or greater, about0.8 or greater, or about 0.9 or greater compared to a Krumbein and Slosschart. The ceramic particles can have an average roundness value ofabout 0.5 or greater, about 0.7 or greater, about 0.8 or greater, orabout 0.9 or greater compared to a Krumbein and Sloss chart.

The ceramic particles can have any suitable density. The ceramicparticles can have a density of at least about 1.5 g/cc, at least about1.7 g/cc, at least about 1.9 g/cc, at least about 2.1 g/cc, at leastabout 2.3 g/cc, at least about 2.5 g/cc, at least about 2.7 g/cc, atleast about 3 g/cc, at least about 3.3 g/cc, or at least about 3.5 g/cc.In one or more exemplary embodiments, the ceramic particles can have adensity of less than 4 g/cc, less than 3.5 g/cc, less than 3 g/cc, lessthan 2.75 g/cc, less than 2.5 g/cc, or less than 2.25 g/cc. For example,the ceramic particles can have a density of about 1.6 g/cc to about 3.5g/cc, about 1.8 g/cc to about 3.2 g/cc, about 2.0 g/cc to about 2.7g/cc, about 2.1 g/cc to about 2.4 g/cc, or about 2.2 g/cc to about 2.6g/cc.

The ceramic particles can have any suitable bulk density or packingdensity. In one or more exemplary embodiments, the ceramic particleshave a bulk density of less than 3 g/cc, less than 2.5 g/cc, less than2.2 g/cc, less than 2 g/cc, less than 1.8 g/cc, less than 1.6 g/cc, orless than 1.5 g/cc. The ceramic particles can have a bulk density ofabout 1 g/cc, about 1.15 g/cc, about 1.25 g/cc, about 1.35 g/cc, orabout 1.45 g/cc to about 1.5 g/cc, about 1.6 g/cc, about 1.75 g/cc,about 1.9 g/cc, or about 2.1 g/cc or more. For example, the ceramicparticles can have a bulk density of about 1.3 g/cc to about 1.8 g/cc,about 1.35 g/cc to about 1.65 g/cc, or about 1.5 g/cc to about 1.9 g/cc.

The ceramic particles can have any suitable surface roughness measuredin accordance with the method disclosed in U.S. Pat. Nos. 8,865,631,8,883,693, and 9,175,210. The ceramic particles can have a surfaceroughness of less than 5 μm, less than 4 μm, less than 3 μm, less than2.5 μm, less than 2 μm, less than 1.5 μm, or less than 1 μm. Forexample, the ceramic particles can have a surface roughness of about 0.1μm to about 4.5 μm, about 0.4 μm to about 3.5 μm, or about 0.8 μm toabout 2.8 μm.

Impinging a plurality of the ceramic particles under a gas-entrainedvelocity onto a flat mild steel target can result in an erosivity of thetarget material. Impinging the gas-entrained ceramic particles at avelocity of about 160 meters per second (m/s) onto the flat mild steeltarget can result in an erosivity of about 0.01 milligrams lost from theflat mild steel target per kilogram of proppant contacting the target(mg/kg), about 0.05 mg/kg, about 0.1 mg/kg, about 0.5 mg/kg, about 1mg/kg, or about 2 mg/kg to about 5 mg/kg, about 7 mg/kg, about 10 mg/kg,about 12 mg/kg, or about 15 mg/kg. Impinging the gas-entrained ceramicparticles at a velocity of about 200 m/s onto the flat mild steel targetcan result in an erosivity of about 0.01 mg/kg, about 0.05 mg/kg, about0.1 mg/kg, about 0.5 mg/kg, about 1 mg/kg, or about 2 mg/kg to about 5mg/kg, about 7 mg/kg, about 10 mg/kg, about 12 mg/kg, or about 15 mg/kg.Impinging the gas-entrained ceramic particles at a velocity of about 260m/s onto the flat mild steel target can result in an erosivity of about1 mg/kg, about 5 mg/kg, about 10 mg/kg, about 20 mg/kg, about 40 mg/kg,or about 60 mg/kg to about 65 mg/kg, about 70 mg/kg, about 80 mg/kg,about 90 mg/kg, or about 100 mg/kg.

The ceramic particles can have any suitable porosity. The ceramicparticles can have an internal interconnected porosity from about 1%,about 2%, about 4%, about 6%, about 8%, about 10%, about 12%, or about14% to about 18%, about 20%, about 22%, about 24%, about 26%, about 28%,about 30%, about 34%, about 38%, about 45%, about 55%, about 65%, orabout 75% or more. In several exemplary embodiments, the internalinterconnected porosity of the ceramic particles is from about 5% toabout 75%, about 5% to about 15%, about 10% to about 30%, about 15% toabout 35%, about 25% to about 45%, about 30% to about 55%, or about 35%to about 70%.

The ceramic particles disclosed herein can be used in any suitable solarpower application, such as in a solar power tower. FIG. 3 shows aschematic illustration of an elevation view of a solar power tower 308in a field of heliostats 310. With reference to FIG. 3 , an example of asolar energy-based power generation system 300 can include a solarreceiver 302 for receiving solar radiation reflected thereonto by one ormore heliostats 304 for the purpose of heating a falling curtain of theceramic particles disclosed herein located inside the solar receiver302. The solar receiver 302 can be located at or proximate to the top ofa single solar power tower 308, or at some other location, for example(not shown), if an intermediate reflector is used to bounce lightreceived at the top of a tower down to a receiver located at groundlevel. Solar receiver 302 can include a solid particle process flowloop.

Each heliostat 304 in the field 310 can track the sun so as to reflectlight onto the receiver 302 in the tower 308. Heliostats can be arrayedin any suitable manner, for example their spacing and positioning can beselected to provide optimal financial return over a life cycle accordingto predictive weather data and at least one optimization goal such astotal solar energy utilization, energy storage, electricity production,or revenue generation from sales of electricity.

FIG. 4 shows a schematic illustration of a solid particle process flowloop 400 for the solar power tower 308. The solid particle process flowloop 400 can include a solid particle receiver 402 in which solarradiation from the heliostats 304 contact falling ceramic particles 404to provide heated falling ceramic particles 406. The heated fallingceramic particles 406 can then be collected in a high temperature solidsstorage vessel 408. The collected ceramic particles can then be directedto one or more heat exchangers 410, such as a direct contact heatexchanger, in which the heat trapped in the collected ceramic particlescan be transferred to a heat exchange medium to provide cooled ceramicparticles. The cooled ceramic particles can be collected in one or morelow temperature solids storage vessels and then can be recycled to thesolid particle receiver 402.

After exposure to heat in the solar power tower 308, the MnO present inthe ceramic particles can darken by shifting from Mn²⁺ to Mn³⁺,resulting in darkened ceramic particles. A cumulative exposure of solarheat energy in the solar power tower 308, for example at a temperatureof about 100° C., about 200° C., about 300° C., or about 400° C. toabout 600° C., about 700° C., about 800° C., or about 1,000° C. or morefor about 10 hours to about 10,000 hours, about 50 hours to about 5,000hours, or from about 250 hours to about 2,500 hours, can darken theceramic particles by any suitable amount. The cumulative exposure ofsolar heat energy to the ceramic particles in the solar power tower 308can darken the ceramic particles to produce darkened ceramic particleshaving a Munsell Value of less than 7, less than 6, less than 5, or lessthan 4 based on the Munsell Color System found in the Munsell Book ofColor. “Value”, or “lightness”, varies vertically along the color solid,from black (value 0) at the bottom, to white (value 10) at the top ofthe vertical axis of the Munsell Color System. Neutral grays lay alongthe vertical axis between black and white. In one or more exemplaryembodiments, subjecting the MnO containing ceramic particles to thecumulative exposure of solar heat energy in the solar power tower 308can reduce the Munsell Value of the ceramic particles from greater than6, greater than 5, or greater than 4 to less than 5, less than 4, orless than 3. Exposure of the MnO containing ceramic particles to solarheat energy in the solar power tower 308 can reduce a Munsell Value ofthe ceramic particle by at least about 0.1, at least about 0.3, at leastabout 0.5, at least about 0.7, or at least about 1. For example,subjecting the MnO containing ceramic particles to the cumulativeexposure of solar heat energy in the solar power tower 308 can reducethe Munsell Value of the ceramic particles by about 0.1, about 0.3,about 0.5, about 0.7, or about 1 to about 1.2, about 1.5, about 2, orabout 2.5 or more.

In one or more exemplary embodiments, the ceramic particles can containFeO and MnO. After exposure to solar heat energy in the solar powertower 308, the FeO present in the ceramic particles can lighten byshifting from Fe²⁺ to Fe³⁺, while the MnO present in the ceramicparticles can darken by shifting from Mn²⁺ to Mn³⁺, which can result inceramic particles having a stable, or substantially unchanged, MunsellValue after the cumulative exposure to heat in the solar power tower308. For example, the Munsell Value of the FeO and MnO containingceramic particles can be less than 6, less than 5, less than 4, or lessthan 3 before and/or after subjecting the ceramic particles to thecumulative exposure of solar heat energy in the solar power tower 308.Exposure of the ceramic particles containing both MnO and FeO to solarheat energy in the solar power tower 308 can reduce a Munsell Value ofthe ceramic particle by at least about 0.1, at least about 0.3, at leastabout 0.5, at least about 0.7, or at least about 1. For example,subjecting the ceramic particles containing both FeO and MnO to thecumulative exposure of solar heat energy in the solar power tower 308can reduce the Munsell Value of the ceramic particles by about 0.1,about 0.3, about 0.5, about 0.7, or about 1 to about 1.2, about 1.5,about 2, or about 2.5 or more.

While the present disclosure has been described in terms of severalexemplary embodiments, those of ordinary skill in the art will recognizethat embodiments of the present disclosure can be practiced withmodification within the spirit and scope of the appended claims.

The present disclosure has been described relative to a severalexemplary embodiments. Improvements or modifications that becomeapparent to persons of ordinary skill in the art only after reading thisdisclosure are deemed within the spirit and scope of the application. Itis understood that several modifications, changes and substitutions areintended in the foregoing disclosure and in some instances some featuresof the present disclosure will be employed without a corresponding useof other features. Accordingly, it is appropriate that the appendedclaims be construed broadly and in a manner consistent with the scope ofthe present disclosure.

What is claimed is:
 1. A ceramic particle for use in a solar powertower, comprising: a ceramic material formed from a mixture comprising araw material and MnO, wherein the raw material comprises kaolin orbauxite or a mixture thereof, and wherein the ceramic material furthercomprises: about 0.01 wt % to about 10 wt % MnO, about 0.1 wt % to about20 wt % Fe₂O₃, and about 0.01 wt % to about 10 wt % Mn₂O₃; and whereinthe ceramic particle has a size of about 8 mesh to about 170 mesh. 2.The ceramic particle of claim 1, wherein the mixture further comprisesabout 0.01 wt % to about 20 wt % FeO.
 3. The ceramic particle of claim1, wherein the ceramic material further comprises about 0.1 wt % toabout 20 wt % FeO.
 4. The ceramic particle of claim 1, wherein theceramic particle has a density of at least about 1.5 g/cc.
 5. Theceramic particle of claim 1, wherein the ceramic particle has a surfaceroughness of less than 5 μm.
 6. The ceramic particle of claim 1, whereinthe ceramic particle has a spherical shape.
 7. The ceramic particle ofclaim 5, wherein exposure of the ceramic particle to solar heat energyin the solar power tower reduces a Munsell Value of the ceramic particleby at least about 0.1.
 8. The ceramic particle of claim 1, wherein themixture is formed into green pellets that are calcined to provide theceramic material.
 9. The ceramic particle of claim 1, wherein themixture is formed into green pellets that are dried and then sintered toprovide the ceramic material.
 10. A solar power tower comprising theceramic particle of claim
 1. 11. A method of manufacturing ceramicparticles for use in a solar power tower, comprising: introducing a rawmaterial and manganese oxide to a mixer, the raw material comprisingabout 0.1 wt % to about 50 wt % silica and about 30 wt % to about 99 wt% alumina; stirring the raw material and manganese oxide to form a dryhomogeneous particulate mixture having an average particle size of lessthan about 15 microns; introducing water to the mixer to contact the dryhomogeneous particulate mixture; stirring the dry homogeneousparticulate mixture with the water to cause formation of green pellets;removing the green pellets from the mixer; drying the green pellets toprovide dried pellets; and sintering the dried pellets to provide aplurality of ceramic particles, wherein the sintering oxidizes a firstportion of the manganese oxide from MnO to Mn₂O₃.
 12. The method ofclaim 11, wherein the raw material comprises alumina, kaolin, or bauxiteor mixtures thereof.
 13. The method of claim 11, further comprisingsubjecting the ceramic particles to solar heat energy in the solar powertower to oxidize a second portion of the manganese oxide from MnO toMn₂O₃.
 14. The method of claim 11, wherein the plurality of ceramicparticles has a Munsell value of at least 6 prior to exposure to solarheat energy in a solar power tower.
 15. The method of claim 14, whereinthe plurality of ceramic particles has a Munsell value of less than 6after exposure to the solar heat energy in the solar power tower. 16.The method of claim 11, wherein sintering comprises subjecting the driedpellets to a temperature between about 1,250° C. to about 1,550° C. forabout 4 minutes to about 20 minutes.
 17. A method of manufacturingceramic particles for use in a solar power tower, comprising:introducing a raw material and manganese oxide to a mixer; introducingabout 0.01 wt % to about 20 wt % FeO to the mixer; stirring the rawmaterial, manganese oxide, and FeO to form a dry homogeneous particulatemixture having an average particle size of less than about 15 microns;introducing water to the mixer to contact the dry homogeneousparticulate mixture; stirring the dry homogeneous particulate mixturewith the water to cause formation of green pellets; removing the greenpellets from the mixer; drying the green pellets to provide driedpellets; and sintering the dried pellets to provide a plurality ofceramic particles, wherein the sintering oxidizes a first portion of themanganese oxide from MnO to Mn₂O₃.
 18. The method of claim 17, whereinthe sintering oxidizes a first portion of the iron oxide from FeO toFe₂O₃.
 19. The method of claim 18, wherein a second portion of the ironoxide is oxidized from FeO to Fe₂O₃ upon subjecting the ceramicparticles to solar heat energy in a solar power tower.